1. Field of the Invention
The present invention relates in general to a protection device for a digital multimeter and, more particularly, to a device for prevention of the power supply for a plug insert corresponding to a specified measuring function of the digital multimeter when the specified function is not selected.
2. Description of the Prior Art
As well known to those skilled in the art, digital multimeters are used for measuring resistance, current etc. When a rotary type function switch of a digital multimeter is rotated to select, for example, a function except the 10 ampere (A) function, an input terminal of 10A or a 10A plug insert of the multimeter should be prevented from being applied with voltage. Otherwise, the digital multimeter may be electrically shocked and damaged. In order to achieve the above object, there have been proposed several types of devices for protection of digital multimeters, such as a device for signaling a plug misinsertion and a device for exclusively opening a plug insert corresponding to a selected function.
With reference to FIG. 1A, there is shown a conventional device for signaling the plug misinsertion. This device signals the misinsertion of the input plug of the digital multimeter when the input plug is unconsciously inserted into the 10A plug insert of the multimeter while disregarding the selection of a function except the 10A function. The above signaling device comprises a signaling circuit 110 which is formed on a printed circuit board (PCB) and in which alarm means 111 and a power source 112 are electrically connected to each other. The ON/OFF operation of the signaling circuit 110 is carried out by both a contact spring 105a mounted on a lower surface of the function switch and a plug insert or an input core 117. This input core 117 includes two terminals 117a and 117b therein and causes the circuit 110 to be closed when the input plug 115 is inserted into the input core 117 as shown in FIG. 1B. The contact spring 105a is mounted on a predetermined position of the lower surface of the function switch such that it reliably closes the alarming circuit 110 when a function except the 10A function is selected by the function switch but opens the circuit 110 when the 10A function is selected by the switch.
When the input plug 115 is inserted into the input core 117 of the device under the condition that the function except the 10A function was selected by the function switch, the signaling circuit 110 is closed and signals, using its alarm means 111, the misinsertion of the input plug 115 into the input core 117. In order to generate the alarm, the contact spring 105a of the function switch as well as the two terminals 117a and 117b of the input core 117 must be turned on at the same time in order to close the alarming circuit 110.
Turning to FIGS. 2A and 2B, there is shown a device for exclusively opening an input plug insert corresponding to a selected desired function. The device comprises a shielding plate 123 which is hinged to a lower surface of a top case 120 of the digital multimeter such that it is rotated when a rotary type function switch 122 is rotated for selection of a desired function. That is, the shielding plate 123 is rotated by rotation of the function switch 122, otherwise stated, this plate 123 cooperates with this switch 122. When the rotary function switch 122 is rotated and selects the 10A function, an input core 124 corresponding to the 10A function is exposed to the outside while another input core 126 is shielded by the shielding plate 123 as shown in FIG. 2B. On the other hand, when the rotary function switch 122 is rotated and selects a function except the 10A function, the input core 124 corresponding to the 10A function is shielded by the shielding plate 123 while the other input core 126 is exposed to the outside as shown in FIG. 2A.
However, the above devices for protection of digital multimeters have their respective problems described hereinbelow.
The plug misinsertion signaling device of FIGS. 1A and 1B, while generating an alarm sound in the case of a plug misinsertion, nevertheless has a problem in that it cannot previously prevent the plug misinsertion. In addition, when the input plug is removed from a voltage measuring input core and inserted into the 10A current measuring input core in the state that the voltage measuring function was selected by the function switch, the voltage is applied to the digital multimeter before the alarm sound is generated by the alarm means 111, thus causing electric shock and damage to the digital multimeter.
Meanwhile, the device having the rotatable shielding plate 123 of FIGS. 2A and 2B, while preventing misinsertion of the input plug, nevertheless has a problem in that it generates no alarm sound. The practical use of this device causes another problem. That is, when setting a desired function, the input plug having been inserted in an input core must be removed from that input core before rotation of the function switch and, thereafter, this input plug must be inserted into another input core corresponding to the desired function after rotation of the function switch for selection of the desired function. Thus, this device requires the user to remove the input plug from an input core and to insert the plug into another insert core on all occasions of operation of the function switch.